Exploration for oil and natural gas reserves led drillers offshore many years ago and as offshore exploration continues, offshore producers find themselves in deeper and deeper waters. While those waters may bring the reserves they seek, the offshore producers are also faced with stronger currents threatening the structural integrity of their risers, pipelines, and other elongated and cylindrical components involved in oil and gas production.
Stresses on the pipes or other structural components immersed in fluid, such as drilling risers, production risers, pipelines, structural tendons, etc. greatly increase as the velocity of the current increases and the stresses are magnified as the depth of the water and length of the risers at the well location increases. When operating rigs in high current areas, the riser is exposed to currents that can cause at least two kinds of stresses. The first is caused by vibration resulting from vortices shed off a component when fluid flows by. That vibration, occurring perpendicular to the current, is referred to as “vortex-induced vibration,” or “VIV.” When water flows past the riser, it may cause vortices to be alternatively shed from each side of the riser when Reynolds Numbers reach a certain range. If the frequency of this harmonic load is near the resonant frequency of the riser, large vibrations transverse to the current can occur. The second type of stress is caused by the drag forces that push the riser in the direction of the current due to the riser's resistance to fluid flow. The drag forces are significantly amplified by the vortex-induced vibrations of the riser, i.e. a vibrating pipe. A riser pipe that is vibrating due to vortex shedding will disrupt the flow of water around it more than a stationary riser or a non-vibrating pipe. This results in more energy transfer from the current to the riser, and hence more drag.
The movement of oil and gas exploration, development and production into deep and ultra-deep waters has created unique engineering challenges requiring innovative engineering solutions. One particular challenge is the vortex-induced-vibrations (VIV) of long drilling and production risers. As discussed above, when long elements such as subsea pipelines, risers, tendons, umbilicals and cables are affected by relatively strong currents over extended lengths along the element, the currents may cause vortices to be shed from the sides of the element in an alternating manner which can induce VIV. The resulting vibration increases drag, reduces fatigue life and left unchecked may lead to the failure of the marine element or its supports.
Shrouds, strakes and fairings have traditionally been added to risers and other submerged pipes in order to minimize the current-induced stresses on these pipes. Strakes and shrouds can be effective regardless of the current orientation, but they tend to increase the drag acting on the riser. By contrast, fairings are generally more efficient in reducing drag and VIV. Fairings generally comprise streamlined shaped bodies (such as airplane wings) that weathervane or rotate about the riser maintaining positions substantially aligned with the water current. Fairings generally reduce vortex-induced forces and minimize drag on the riser by reducing or breaking up the low pressure areas that exist down-current of the riser.
One example of a fairing is found in U.S. Pat. No. 4,474,129 that discloses a fairing removably mountable on risers equipped with buoyancy modules that has a tail tapering aft and a fin positioned after the tail. This fairing completely surrounds the riser and is fastened together at the back portion of the fairing. Another example of a fairing is found in U.S. Pat. No. 4,398,487 which describes a streamlined symmetrical structure having a nose portion, a tail portion and two opposed side portions. This fairing is formed as two shell halves that completely surround the riser and are connected at the front end of the nose by quick release fasteners and at the end of the tail portion by hinges. U.S. Pat. No. 5,410,979 describes a small fixed teardrop-shaped fairing that surrounds a riser and is fixed to the riser so as to not rotate. U.S. Pat. No. 6,048,136 describes a fairing that is installed on a drilling riser in combination with a synthetic foam buoyancy module. This fairing is formed as two shell halves that surround the riser and attach at the front and back portions of the fairing. A rotating fairing is described in U.S. Pat. No. 6,067,922 as including a copper element mounted in the annular region of the fairing to discourage marine growth. This fairing is formed as a single piece that completely surrounds the riser and is attached at the tail or flange portion with bolts. An ultrashort fairing described in U.S. Pat. No. 6,223,672 is shown in FIG. 2. This fairing 2 has a pair of shaped sides 4A, 4B departing from the circular profile of a marine riser and converging at a trailing edge 6. It should be noted that all of the above described fairings are constructed in predetermined lengths and a plurality of fairings are positioned along the length of any particular riser.
While fairings can be effective for reducing VIV, a number of problems still exist with the prior art fairings. As illustrated in the prior art, fairings have become more and more complex in design, they often require a large number of parts, and as such, they have become more costly to produce and maintain. Generally, fairings must be secured to the elongated component by bands, bolts or other fasteners that may fail. Further, the use of such fasteners adds to the cost and labor associated with the fairing's use. Additionally, corrosion and marine growth frequently causes the rotational elements of a fairing to seize up so that it can no longer rotate and properly align with the current. Such a concern has often resulted in fairings being used only on risers or other components that remain on the risers only a short period of time, leaving those in the industry to rely upon less effective VIV reduction means such as fixed-fin vortex strakes for more permanently fixed components.
Although strakes with certain fin heights and fin periods can reduce the amplitude of the VIV induced motion by more than 90% and have been proven to be effective tools, a lower drag solution would be desirable. While conventional teardrop fairings are effective in reducing both drag and VIV, users report that these devices are subject to a “galloping” motion. The causes of this galloping motion remain unclear. Therefore, there remains a need for an improved VIV suppression device that reduces vibration, that does not increase drag and is resistant to galloping motions.
Fairings are typically applied to drilling and/or production risers in one of two ways. In one manner of installation, fairings are placed on the riser after it is in place, suspended between the platform and the ocean floor, in which divers or submersible vehicles (referred to as ROVs) are used to fasten the multiple fairings around the riser. A second method of installation is carried out as the riser is being assembled on a vessel and installed. In this method the fairings are fastened to the pipe as lengths of pipes are fitted together to form the riser. This method of installation is typically performed on a specially designed vessel, called an S-Lay, J-Lay barge or Reel Lay barge. An S-Lay barge is one that has a declining ramp, positioned along a side or rear of the vessel and descending below the ocean's surface, that is equipped with rollers (referred to as a “stinger”). As the lengths of pipe are fitted together, fairings are attached to the connected pipe sections before the pipe is rolled down the ramp and into the ocean. One of the problems of installing fairings in this manner is that when the fins of the fairing rotate over the rollers on the ramp, the fins frequently become damaged by the rollers. In this method of installation, the completed riser laid on the ocean floor, then is pulled up to a vertical position when it reaches the appropriate length and is attached to the surface platform and the well head on the ocean floor.
It would be advantageous to provide a relatively lightweight, resilient fairing that can be easily placed on a riser rather than having to be fastened around the entire circumference.
It would be advantageous to provide a fairing that reduces vibration, that does not increase drag and is resistant to galloping motions.